Method and apparatus for channel estimation and detection in MIMO system

ABSTRACT

The prevent invention provides a method for performing channel estimation and detection in a Multiple-Input-Multiple-Output (MIMO) system, including: generating a demodulation reference signal, DM-RS, pattern for an extended cyclic prefix (CP); and estimating and detecting channel performance based on the generated DM-RS signal pattern. The DM-RS pattern supports from rank 1 to rank 8 patterns for 8 layer transmission. A transmitter, a receiver and a system thereof are also provided. The solution of the present invention improves channel estimation accuracy, saves channel estimation implementation and implementation complexity at terminals.

FIELD OF THE INVENTION

The present invention relates to field of wireless communication, and more particularly to coherent detection with demodulation reference signal (DM-RS) for the extended cyclic prefix (extended CP) in the Evolved Universal Terrestrial Radio Access (E-UTRA) system, which is also commonly referred to as the Long-Term Evolution (LTE) of the widely deployed WCDMA systems.

BACKGROUND OF THE INVENTION

Long Term Evolution (LTE) is currently under discussion as a next generation mobile communication system of the UMTS system. LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink. The 3GPP work on LTE is also referred to as E-UTRAN (Evolved Universal Terrestrial Access Network). In order to support high data rates, LTE allows for a system bandwidth of up to 20 MHz. LTE is also able to operate in different frequency bands and can operate in at least Frequency Division Duplex (FDD) and Time Division Duplex (TDD). The modulation technique or the transmission scheme used in LTE is known as OFDM (Orthogonal Frequency Division Multiplexing). LTE-advanced can be viewed as a future release of the LTE standard and since it is an evolution of LTE, backward compatibility is important so that LTE-advanced can be deployed in spectrum already occupied by LTE. In both LTE and LTE-advanced radio base stations known as eNBs or eNodeBs—where e stands for evolved—, multiple antennas with beamforming technology can be adopted in order to provide high data rates to user equipments. Thus, LTE and LTE-advanced are an example of Multiple-Input-Multiple-Output (MIMO) radio systems. Another example of a MIMO based system is Worldwide Interoperability for Microwave Access (WiMAX) system.

As defined in 3GPP LTE, RS is multiplexing with data into OFDM time-frequency grid. In Rel-8, three kinds of downlink reference signals are defined in terms of different functionality, i.e. cell-specific reference signal, UE-specific reference signal and MBSFN reference signal. Here, the first two kinds of reference signals are focused.

Cell-specific reference signals (also called CRS) are responsible for both channel measurement and data demodulation in Rel-8 for the downlink transmission modes except mode 7 (i.e. single layer beamforming). UE terminals will use CRS to calculate Channel Quality Indication/Precoding Matrix Index/Rank Indicator (CQI/PMI/RI) and also do data demodulation by performing channel estimation. Total 4 CRS are defined to support up to 4 layers transmission, i.e. antenna ports 0-3, as shown in FIGS. 1A, 1B, 1C (normal CP only), the detailed description can be found in the standard document, “3GPP TS 36.211 V8.6.0 (2009-03)”.

UE-specific reference signals (also called dedicated reference signal, or DRS) are introduced only for the transmission mode 7 in Rel-8, i.e. single layer beamforming. DRS are precoded and share the same precoder as data. In addition, DRS is responsible for channel demodulation only, which means channel estimation for data demodulation will be performed based on DRS. Only single DRS port is defined in Rel-8 to support single layer transmission, i.e. antenna port 5. The DRS pattern for normal CP and extended CP is defined respectively, as shown in FIG. 2 (normal CP only). The detailed description can be found in the standard document, “3GPP TS 36.211 V8.6.0 (2009-03)”.

In “3GPP, RAN1_(—)58bis Chairman notes, RAN1_(—)58bis, Oct. 12-16, 2009, Miyazaki, Japan”, DM-RS design for normal CP, as shown in FIG. 3, acts as up to four layers DM-RS pattern. Some characteristics are summarized here: total 12 resource elements per layer are agreed as DM-RS overhead (either marked as 1 or 2 in FIG. 3). Two CDM groups (marked as 1 and 2) occupy different subcarriers in frequency domain. Each CDM group uses 6 orthogonal code covers (OCC) with each length of 2 to multiplex up to two layers. This is a non-staggered version, i.e. 1st DM-RS cluster and 2nd DM-RS cluster take the same subcarrier.

In order to fulfill LTE-advanced downlink spectral efficiency, 30 bps/Hz, up to 8 layer transmission should be supported using some kind of advanced antenna configuration, e.g. 8×8 high-order MIMO. If similar design of up to 8 cell-specific reference signals is introduced, system overhead will be inevitably increased and accordingly throughput performance will be decreased seriously. Accordingly, there is a need for generating a DM-RS pattern for extended CP supporting from rank 1 to rank 8 for both normal subframe and Downlink Pilot Time Slot (DwPTS). Based on this consideration, it is proposed CSI-RS (namely cell-specific reference signal) is targeted for channel measurement only while DM-RS (namely UE-specific reference signal) is targeted for channel demodulation only. CSI-RS can be accessed by all UE terminals in a cell and DM-RS can be accessed by the UE only on the allocated resource blocks (RB).

On DM-RS pattern for extended CP, it is known that extended CP is not supported in conjunction with transmission mode 8 (i.e., dual layer beamforming mode). The use of extended CP is expected for channels with larger time dispersion, or equivalently for channels experiencing much more frequency selectivity as compared to channels where normal CP is used, e.g. Vehicular B channel (VehB). Therefore, there is a need for generating a DM-RS pattern for extended CP to guarantee and improving detection performance in such a channel.

In TDD, DwPTS has different length according to different special subframe configurations, which results in quite a lot of designs at UE side. Therefore, there is a need for generating a DM-RS pattern for extended CP to reduce UE implementation complexity.

The pattern designed for normal CP has been accepted well. Some design principles have been settled down. Therefore, there is a need for generating a DM-RS for extended CP which has smooth and close design for extended CP to further save UE implementation and standard effort.

SUMMARY OF THE INVENTION

The present invention provides a method for performing channel estimation and detection in a Multiple-Input-Multiple-Output, MIMO, system, including: generating a demodulation reference signal, DM-RS, pattern for an extended cyclic prefix, CP; and estimating and detecting channel performance based on the generated DM-RS signal pattern. The DM-RS pattern includes rank 5-8 pattern which supports for 8 layer transmission.

The present invention also provides a transmitter including, a demodulation reference signal pattern generator for generating a DM-RS pattern for an extended cyclic prefix. The DM-RS pattern includes rank 5-8 pattern which supports for 8 layer transmission.

In a further aspect of the invention, a receiver is provided including, a channel estimation module, adapted to estimating channel performance by using a demodulation reference signal, DM-RS, pattern for an extended cyclic prefix; and a channel detection module, adapted to detecting coherently by using estimated channel state information, CSI, contained in the DM-RS. The DM-RS pattern includes rank 5-8 pattern which supports for 8 layer transmission.

In another aspect of the invention, a system including the transmitter and the receiver as above is provided.

In conclusion, this invention proposes staggered patterns with a bit higher RS overhead for extended CP than normal CP from rank-1 to rank-8. Channel estimation and detection by using the patterns of the invention with a bit higher RS overhead in frequency domain can provide good capability of capturing fast frequency variations to improve channel estimation accuracy, which is typically suitable to extended CP case. In addition, the patterns of the invention can keep a subset design as much as possible to save channel estimation implementation at terminals. Furthermore, when in a TDD system, the patterns of the invention can save standard effort and implementation complexity as well since a unified pattern is proposed for all DwPTS lengths. The designed patterns can extensively be applied for features, e.g. higher-order MIMO, Multi-user MIMO (MU-MIMO), Coordinated Multipoint Transmission (CoMP) and possibly relaying, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following section, the invention will be described with reference to exemplary embodiments illustrated in the figures, in which:

FIGS. 1A, 1B, 1C show shows an exemplary diagram illustrating up to 4 downlink cell-specific reference signals (normal CP) in prior art;

FIG. 2 shows an exemplary diagram illustrating one downlink UE-specific reference signal (normal CP);

FIG. 3 shows an exemplary diagram illustrating up to 4 layers DM-RS pattern for normal CP in prior art;

FIG. 4 shows an exemplary structural diagram illustrating a wireless system with a DM-RS pattern according to an embodiment of the present invention;

FIG. 5 shows a flowchart illustrating a method for channel estimation and detection by using a DM-RS pattern according to an embodiment of the present invention;

FIG. 6 shows an exemplary extended CP rank 1-4 pattern for both normal subframe (left) and DwPTS (right) according to an embodiment of the present invention;

FIG. 7 shows an exemplary extended CP rank 5-8 pattern (option-1) with OCC=4 according to an embodiment of the present invention;

FIG. 8 shows an exemplary extended CP rank 5-8 pattern (Option-2) with OCC=4 according to an embodiment of the present invention; and

FIG. 9 shows an exemplary extended CP rank 5-8 pattern (Option-3) with OCC=2 according to an embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention will be described by referring to the accompanying drawings.

Reference signal (RS), as also called predefined training sequence/signal, is extensively applied in wireless communication systems with the aim to improve detection performance. Namely, in a wireless communication system, RS carried by transmitter is responsible for receiver to do channel estimation. Coherent detection by using the estimated channel state information (CSI) can be applied, which is considered an efficient method to improve performance.

Referring to FIG. 4, a wireless communication system with reference signals includes at least a transmitter and a receiver is provided. RS is predefined between transmitter and receiver. The transmitter includes a DM-RS pattern generator for generating a DM-RS pattern for an extended cyclic prefix; at the receiver, channel estimation is applied first by using RS targeting to do coherent detection for data recovery. The receiver includes a channel estimation module for estimating channel performance by using the DM-RS pattern for an extended cyclic prefix; and a channel detection module for detecting coherently by using estimated channel state information, CSI, contained in the DM-RS. The DM-RS pattern supports from rank 1 to rank 8 for 8 layer transmission.

FIG. 5 shows a method for performing channel estimation and detection in an MIMO system. The method includes the following steps:

In step 10, a demodulation reference signal pattern for an extended cyclic prefix is generated from the DM-RS pattern generator at the transmitter.

In step 20, channel estimation and detection is performed respectively by a channel estimation module and a channel detection module, at the receiver, in dependence on the generated DM-RS signal pattern.

The DM-RS pattern includes rank 5-8 patterns which supports for 8 layer transmission.

Hereinafter, two DM-RS patterns cases, i.e. rank 1-4 pattern and rank 5-8 pattern are provided in detail.

Case 1: Rank 1-4 Pattern

An embodiment of the present invention provides a DM-RS pattern for the support of up to four layers transmission, as shown in FIG. 6. The detection performance of this pattern can be guaranteed for extended CP case. In this figure, 3 or 2 control symbols are given as an example for normal subframe and DwPTS, respectively.

In this patter, the DM-RS overhead of 16 REs per layer is used, which is a bit higher than 12 REs for normal CP (see FIG. 3). 16 REs per layer is selected for making a best tradeoff between channel estimation/detection performance and overhead. This may solve the problem that channel estimation accuracy is degraded because of highly frequency selective fading.

A staggered structure is proposed to further improve performance. This can benefit to detailed channel estimation algorithm, e.g. 2D-MMSE filter method, which is extensively used for DM-RS based transmission modes.

A punctured version of normal subframe is simply proposed for all DwPTS cases, i.e. DwPTS with 8, 9 or 10 OFDM symbols. This can ease UE implementation since only one DM-RS pattern is used for channel estimation when applicable to different DwPTS case. Of course, this is not suitable to the case of DwPTS with 3 OFDM symbols since no data transmission is expected

The same as normal CP case, CDM+FDM is basically used to multiplex up to four layers. This maintains good consistency of DM-RS design between normal CP and extended CP. In another word, design principles can be shared to a great extent.

A good subset design is kept since per layer channel estimation is the same regardless of different rank pattern. For instance, layer 1 channel estimation implementation is the same in rank-1 and rank-4 pattern.

Case 2: Rank 5-8 Pattern

Based on this rank 1-4 pattern, three preferred optional DM-RS patterns for rank 5-8 patterns are provided to support up to 8 layer transmission, as shown in FIGS. 7-9. This may give a smooth and simple pattern design.

FIG. 7 shows Option-1 for DM-RS pattern for extended CP where OCC=4. The Option-1 re-uses the resource elements reserved for DM-RS in rank 1-4 pattern, which to some extent eases channel estimation implementation at UE. This option keeps the same characteristics of rank 1-4 pattern, i.e. 16 REs per layer, staggered structure, a subset design and unified pattern for all DwPTS cases. The difference is to use OCC with length of 4 for each CDM group, where OCC is constructed in time-frequency domain for both normal subframe and DwPTS, as shown in FIG. 7. The advantage is to make a soft extension by modifying OCC length from 2 to 4.

For normal subframe, the DM-RS pattern in Option-1 includes 16 REs per layer, and the 16 REs for CDM group 1 includes resource elements at R(i, f) in a radio frame, where f=2, 5, 8, 11 when i=5 or 6, and f=3, 6, 9, 12 when i=11 or 12; for CDM group 2, f=1, 4, 7, 10 when i=5 or 6, and f=2, 5, 8, 11 when i=11 or 12, where i represents the i-th symbol in time-domain direction in a radio frame, and f represents the f-th subcarrier in frequency-domain direction in a radio frame.

For DwPTS with 8, 9 or 10 OFDM symbols which is a punctured version of the normal subframe, the DM-RS pattern in Option-1 includes 8 REs per layer, and the 8 REs for CDM group 1 includes resource elements at R(i, f) in a radio frame, where f=2, 5, 8, 11 when i=5 or 6; for CDM group 2, f=1, 4, 7, 10 when i=5 or 6; and REs with i=9, 10, 11, 12 and f=1 to 12 are punctured.

FIG. 8 shows Option-2 for DM-RS pattern for extended CP where OCC=4. The Option-2 provides another pattern with OCC length of 4 as compared to option-1. So, some similar characteristics are maintained, e.g. 16 REs per layer, staggered structure, a subset design and unified pattern for all DwPTS cases. The difference is OCC with length of 4 is constructed in 4 adjacent resource elements. The advantage is this option can efficiently overcome the negative impact of Doppler in a high mobility environment since orthogonality between used OCCs can be relatively kept well.

For normal subframe, the DM-RS pattern in Option-2 includes 16 REs per layer, and the 16 REs for CDM group 1 includes resource elements at R(i, f) in a radio frame, where f=4, 5, 10, 11 when i=5 or 6, and f=2, 3, 8, 9 when i=11 or 12; for CDM group 2, f=1, 2, 7, 8 when i=5 or 6, and f=5, 6, 11, 12 when i=11 or 12, where i represents the i-th symbol in time-domain direction in a radio frame, and f represents the f-th subcarrier in frequency-domain direction in a radio frame.

For DwPTS with 8, 9 or 10 OFDM symbols which is a punctured version of the normal subframe, the DM-RS pattern in Option-2 includes 8 REs per layer, and the 8 REs for CDM group 1 includes resource elements at R(i, f) in a radio frame, where f=4, 5, 10, 11 when i=5 or 6; for CDM group 2, f=1, 2, 7, 8 when i=5 or 6; and REs with i=9, 10, 11, 12 and f=1 to 12 are punctured.

FIG. 9 shows Option-3 for DM-RS pattern for extended CP where OCC=2. The Option-3 provides different solution as compared to option-1/2. Total 4 CDM groups are proposed and OCC with length of 2 is proposed since up to two layers is multiplexed in a CDM group. This option has very good capability of maintaining orthogonality between used OCCs due to OCC length of 2. Less DM-RS overhead of 8 REs per layer is proposed since in extended CP, the application of more than 4 layer transmission under like VehB channel is very low. So, this overhead is good enough to obtain good detection performance under TU like channel. Still, some similar characteristics are maintained, e.g. staggered structure, a subset design and unified pattern for all DwPTS cases.

For normal subframe, the DM-RS pattern in Option-3 includes 8 REs per layer, and the 8 REs for CDM group 1 includes resource elements at R(i, f) in a radio frame, where f=5, 11 when i=5 or 6, and f=3, 9 when i=11 or 12; for CDM group 2, f=4, 10 when i=5 or 6, and f=2, 8 when i=11 or 12; for CDM group 3, f=2, 8 when i=5 or 6, and f=6, 12 when i=11 or 12; for CDM group 4, f=1, 7 when i=5 or 6, and f=5, 11 when i=11 or 12; where i represents the i-th symbol in time-domain direction in a radio frame, and f represents the f-th subcarrier in frequency-domain direction in a radio frame.

For DwPTS with 8, 9 or 10 OFDM symbols which is a punctured version of the normal subframe, the DM-RS pattern in Option-3 includes 4 REs per layer, and the 4 REs for CDM group 1 includes resource elements at R(i, f) in a radio frame, where f=5, 11 when i=5 or 6; for CDM group 2, f=4, 10 when i=5 or 6; for CDM group 3, f=2, 8 when i=5 or 6; for CDM group 4, f=1, 7 when i=5 or 6; and REs with i=9, 10, 11, 12 and f=1 to 12 are punctured.

In the present invention, the staggered patterns with DM-RS overhead of max 16 REs per layer for extended CP are provided. Three preferred optional patterns are proposed for rank 5-8 design. It may be observed that the foregoing DM-RS patterns for extended CP supporting rank 5-8 have the following advantages:

-   -   Good consistency is maintained by adopting the same design as         normal CP, i.e. TDM multiplexed with control channel and Rel-8         CRS and using CDM+FDM as the basic multiplexing scheme among         layers.     -   Detection performance based on DM-RS can be guaranteed because         the pattern has good capability of capturing channel frequency         domain variations in the system that extended CP could         experience.     -   A subset design is kept as much as possible to ease UE         implementation.     -   A unified pattern by puncturing the pattern used for normal         subframe is used for all DwPTS lengths, which can further limit         UE implementation complexity increasing too much.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention as defined by the appended claims. The exemplary embodiments should be considered in descriptive sense only and not for purposes of limitation. Therefore, the scope of the invention is defined not by the detailed description of the invention but by the appended claims, and all differences within the scope will be construed as being included in the present invention. 

The invention claimed is:
 1. A method for performing channel estimation and detection in a Multiple-Input-Multiple-Output (MIMO) system, comprising: generating a demodulation reference signal (DM-RS) pattern for an extended cyclic prefix (CP); and estimating and detecting channel performance based on the generated DM-RS signal pattern; wherein the DM-RS pattern includes a rank 5-8 pattern that supports 8-layer transmission, and the DM-RS pattern adopts a DM-RS overhead of a maximum of 16 resource elements per layer for an extended CP.
 2. The method of claim 1, wherein the DM-RS pattern comprises a staggered structure.
 3. The method of claim 1, wherein in the DM-RS pattern, a subset design is kept from rank 1 to rank
 8. 4. The method of claim 1, wherein the DM-RS pattern comprises a punctured version of a normal subframe for different downlink pilot time slot (DwPTS) cases.
 5. The method of claim 4, wherein the DwPTS cases include DwPTS having 8, 9, or 10 orthogonal frequency division multiplex symbols.
 6. The method of claim 1, wherein the DM-RS pattern uses code division multiplexing and frequency division multiplexing to multiplex up to four layers.
 7. A method for performing channel estimation and detection in a Multiple-Input-Multiple-Output (MIMO) system, comprising: generating a demodulation reference signal (DM-RS) pattern for an extended cyclic prefix (CP); and estimating and detecting channel performance based on the generated DM-RS signal pattern; wherein the DM-RS pattern includes a rank 5-8 pattern that supports 8-layer transmission; the DM-RS pattern comprises one of a first DM-RS pattern including a total of 2 code division multiplexing (CDM) groups with an orthogonal code cover (OCC) having a length of 4 used for each CDM group, a second DM-RS pattern including a total of 2 CDM groups with an OCC having a length of 4 used for each CDM group, and a third DM-RS pattern including a total of 4 CDM groups with an OCC having a length of 2 used for each CDM group; for the first DM-RS pattern, the OCC is constructed in a time-frequency domain for both a normal subframe and a downlink pilot time slot (DwPTS); and for the normal subframe, a first DM-RS pattern comprises 16 resource elements (REs) per layer; the 16 REs for a CDM group 1 include resource elements at R(i, f) in a radio frame, where f=2, 5, 8, or 11 when i=5 or 6, and f=3, 6, 9, or 12 when i=11 or 12; for a CDM group 2, f=1, 4, 7, or 10 when i=5 or 6, and f=2, 5, 8, or 11 when i=11 or 12; i represents an i-th symbol in a time-domain direction in the radio frame; and f represents an f-th subcarrier in a frequency-domain direction in the radio frame; for the second DM-RS pattern, the OCC is constructed in 4 adjacent resource elements; and for a normal subframe, the second DM-RS pattern comprises 16 resource elements (REs) per layer; the 16 REs for a CDM group 1 include resource elements at R(i, f) in a radio frame, where f=4, 5, 10, or 11 when i=5 or 6, and f=2, 3, 8, or 9 when i=11 or 12; for a CDM group 2, f=1, 2, 7, or 8 when i=5 or 6, and f=5, 6, 11, or 12 when i=11 or 12; i represents an i-th symbol in a time-domain direction in the radio frame; and f represents an f-th subcarrier in a frequency-domain direction in the radio frame; and for a normal subframe, the third DM-RS pattern comprises 8 resource elements (REs) per layer; the 8 REs for a CDM group 1 include resource elements at R(i, f) in a radio frame, where f=5 or 11 when i=5 or 6, and f=3 or 9 when i=11 or 12; for a CDM group 2, f=4 or 10 when i=5 or 6, and f=2 or 8 when i=11 or 12; for a CDM group 3, f=2 or 8 when i=5 or 6, and f=6 or 12 when i=11 or 12; for a CDM group 4, f=1 or 7 when i=5 or 6, and f=5 or 11 when i=11 or 12; i represents an i-th symbol in a time-domain direction in the radio frame; and f represents an f-th subcarrier in a frequency-domain direction in the radio frame.
 8. The method of claim 7, wherein, for a DwPTS with 8, 9, or 10 orthogonal frequency division multiplexing (OFDM) symbols, which is a punctured version of the normal subframe, the first DM-RS pattern comprises 8 REs per layer; the 8 REs for the CDM group 1 include resource elements at R(i, f) in the radio frame, where f=2, 5, 8, or 11 when i=5 or 6; the 8 REs for the CDM group 2 include resource elements at R(i, f) in the radio frame, where f=1, 4, 7, 10 when i=5 or 6; and REs with i=9, 10, 11, 12 and f=1 to 12 are punctured.
 9. The method of claim 7, wherein for a downlink pilot time slot (DwPTS) with 8, 9, or 10 orthogonal frequency division multiplexing (OFDM) symbols, which is a punctured version of the normal subframe, the second DM-RS pattern comprises 8 REs per layer; the 8 REs for the CDM group 1 include resource elements at R(i, f) in the radio frame, where f=4, 5, 10, or 11 when i=5 or 6; for CDM group 2, f=1, 2, 7, or 8 when i=5 or 6; and REs with i=9, 10, 11, or 12 and f=1 to 12 are punctured.
 10. The method of claim 7, wherein for a downlink pilot time slot (DwPTS) with 8, 9, or 10 orthogonal frequency division multiplexing (OFDM) symbols, which is a punctured version of the normal subframe, the third DM-RS pattern comprises 4 REs per layer; the 4 REs for the CDM group 1 include resource elements at R(i, f) in the radio frame, where f=5 or 11 when i=5 or 6; for the CDM group 2, f=4 or 10 when i=5 or 6; for the CDM group 3, f=2 or 8 when i=5 or 6; for the CDM group 4, f=1 or 7 when i=5 or 6; and REs with i=9, 10, 11, or 12 and f=1 to 12 are punctured.
 11. A transmitter of radio frames including subframes, comprising: a demodulation reference signal (DM-RS) pattern generator configured to generate a DM-RS pattern for an extended cyclic prefix; wherein the DM-RS pattern includes a rank 5-8 pattern that supports 8-layer transmission and includes a total of 2 code division multiplexing (CDM) groups; an orthogonal code cover (OCC) having a length of 4 is used for each CDM group and the OCC is constructed in a time-frequency domain for both a normal subframe and a downlink pilot time slot (DwPTS); for a normal subframe, the DM-RS pattern comprises 16 resource elements (REs) per layer; the 16 REs for a CDM group 1 include resource elements at R(i, f) in a radio frame, where f=2, 5, 8, or 11 when i=5 or 6, and f=3, 6, 9, or 12 when i=11 or 12; for a CDM group 2, f=1, 4, 7, or 10 when i=5 or 6, and f=2, 5, 8, or 11 when i=11 or 12; i represents an i-th symbol in a time-domain direction in the radio frame; and f represents an f-th subcarrier in a frequency-domain direction in the radio frame.
 12. A receiver of radio frames including subframes, comprising: a channel estimator configured to estimate channel performance by using a demodulation reference signal (DM-RS) pattern for an extended cyclic prefix; and a channel detector configured to detect coherently by using estimated channel state information (CSI) contained in the DM-RS pattern; wherein the DM-RS pattern includes a rank 5-8 pattern that supports 8-layer transmission and includes a total of 2 code division multiplexing (CDM) groups; an orthogonal code cover (OCC) having a length of 4 is used for each CDM group and the OCC is constructed in a time-frequency domain for both a normal subframe and a downlink pilot time slot (DwPTS); for a normal subframe, the DM-RS pattern comprises 16 resource elements (REs) per layer; the 16 REs for a CDM group 1 include resource elements at R(i, f) in a radio frame, where f=2, 5, 8, or 11 when i=5 or 6, and f=3, 6, 9, or 12 when i=11 or 12; for a CDM group 2, f=1, 4, 7, or 10 when i=5 or 6, and f=2, 5, 8, or 11 when i=11 or 12; i represents an i-th symbol in a time-domain direction in the radio frame; and f represents an f-th subcarrier in a frequency-domain direction in the radio frame.
 13. A communication system, comprising a transmitter according to claim 11 and a receiver according to claim
 12. 